A solar cell converts solar energy directly to DC electric energy. Generally configured as a photodiode, a solar cell permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier, such as an electron or a hole (a lack of an electron), may be extracted as current. Like most other diodes, a photodiodes is formed by combining p-type and n-type semiconductors to form a junction.
Electrons on the p-type side of the junction within the electric field (or built-in potential) may then be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
Substantially affecting solar cell performance, carrier lifetime (or recombination lifetime) is defined as the average time it takes an excess minority carrier (non-dominant current carrier in a semiconductor region) to recombine and thus become unavailable to conduct an electrical current. Likewise, diffusion length is the average distance that a charge carrier travels before it recombines. In general, although increasing dopant concentration improves conductivity, it also tends to increase recombination. Consequently, the shorter the recombination lifetime or recombination length, the closer the metal region must be to where the charge carrier was generated.
A conventional solar cell is generally configured with a set of front and a rear metal contacts on a silicon substrate doped with a first dopant (commonly boron) forming an absorber region, upon which a second counter dopant (commonly phosphorous), is diffused forming the emitter region, in order to complete the p-n junction. After the addition of passivation and antireflection coatings, the metal contacts (fingers and busbar on the emitter and pads on the back of the absorber) may be added in order to extract generated charge.
In an alternative rear-contact solar cell configuration, all the metal contacts are positioned in an inter-digitated manner on the rear of a doped substrate. That is, interweaving alternating n-type regions and p-type regions.
In general, a low concentration of (substitutional) dopant atoms within a doped region will result in both low recombination (thus higher solar cell efficiencies) and in poor electrical contact to metal electrodes. Conversely, a high concentration of (substitutional) dopant atoms will result in both high recombination (thus reducing solar cell efficiency), and a low resistance ohmic contacts to metal electrodes. Often, in order to reduce manufacturing costs, a single (suboptimal) dopant diffusion is generally used to form an emitter, with a doping concentration selected as a compromise between low recombination and low resistance ohmic contact. Consequently, the resulting solar cell efficiency (the percentage of sunlight that is converted to electricity) is reduced.
Also affecting efficiency is the presence of a BSF (back surface field) in the case of a conventional solar cell configuration, or a FSF (front surface field) in the case of a rear contact solar cell configuration. A surface field is configured to help minimize charge carrier recombination by creating a field that repels minority carriers both from surface regions and from metalized regions on the surfaces. Formed using dopants of the same type as those used in the absorber region, the concentration of dopant atoms in a surface field is generally selected to be higher than that used to dope the absorber region.
In the case of a BSF, because aluminum is often used, the resulting electrical conductivity and electric field characteristics (i.e., field strength and field uniformity) for repelling minority carriers of the BSF may be suboptimal for use in a high efficiency (i.e., >17%) solar cells. Typically, aluminum is applied via screen deposition onto the back of a solar cell, and then co-fired in a belt furnace along with the front side metal contacts (commonly formed from screen printed silver paste). Aluminum atoms (with a substantially lower melting temperature than silicon) tend to diffuse into the silicon substrate and become incorporated into the silicon crystal. However, the thermal expansion coefficient of aluminum α of about 25×106 (1/° C.) is approximately 10 times larger than that of silicon, which in turn may cause significant substrate bow as a result of the firing process. That is, as the substrate thickness decreases and the aluminum thickness increases, the amount of substrate bow will increase making the substrates unsuitable for module construction.
Furthermore, an aluminum BSF may also tend to contribute to solar cell efficiency losses, since any light photon that is transmitted through the wafer and is absorbed by aluminum is unavailable for charge carrier generation.
In view of the foregoing, there is a desire to provide a method of using a set of silicon nanoparticle fluids to control in situ both front surface and rear surface dopant diffusion profiles.